A press oven of this type is disclosed, for example, in EP-A1-231 773. In a press oven of this type, a substantially disk- or flat-cylinder shaped sealing-off piston is pressed downwardly via a press stamp (or piston rod) to apply pressure to a blank that melts as the pressure is exerted thereon. The sealing-off piston is comprised of ceramic and has a diameter that corresponds to the diameter of a feed channel for the blank. Also, the blank is comprised of a ceramic whose consistency and material properties are selected to achieve the desired restoration result.
In connection with such press ovens but, as well, in connection with press ovens that do not have a sealing-off piston—by which, therefore, the press stamp itself applies pressure directly on the blank—the problem exists that the press stamp remains slightly stuck to the ceramic. With respect to the approach disclosed in EP-A1-231 773, a problem additionally exists that the sealing-off piston and the blank intensively interconnect with one another at the deformation temperature of the ceramic and, thus, can only be released following an extensive stress relieving working. One must then practically separate the entire sealing-off piston with a diamond cutter which is time-consuming and wear-intensive. The deployed ceramic masses, in particular with the use of zirconium oxide ceramic, are, in fact, very hard and, in this regard, require a particularly specialized work tool for the separation.
A further problem resides in the fact that the press stamp must exert a considerable pressure to ensure that the ceramic of the blank, which has become soft via the increase in the temperature, penetrates into the branching of the mold cavity in which the restoration pieces should be configured. Upon the beginning of plastification, the pressure to be exerted initially sinks considerably but then, however, increases strongly shortly before the end of the filling of the mold. This leads to a significant increase in the pressure that, in particular, strongly loads the muffle and can lead to the occurrence of spalling locations that detract from the required precision for the production of the dental restoration pieces.
In order to mitigate the effect of this pressure increase, it has already been proposed to deploy a path-/time control that should reduce the pressure at the time of pressure increases and the consequent reduced operation of the press stamp per unit of time. An approach of this type requires, to be sure; a decidedly exact and very rapidly reacting control system for the press operation. Such systems must compensate for, as well, the dead times of the system and are, consequently, expensive.
In order to prevent this, it has heretofore been the practice to operate with a comparatively reduced operational speed. In more recent times, attempts have been undertaken to increase the operational speed in order to improve the cycling time for the preparation and the making available of the dental replacement piece. This requires a still more complex control if a path-/time measurement is to be implemented, to thereby then avoid the feared pressure peaks.
Further in this connection, it has already been proposed to undertake, in lieu of a path-/time measurement, a measurement of the pressure exerted by the press stamp. This approach, however, requires as well a compensation of the dead time in the control system, whereupon such a control system is complex and is, as well, particularly difficult to calibrate.
In this connection, the operational speed of the press stamp is, typically, at a minimum lowered toward the end of the estimated end of the press process. In view of the fact that the positional height of the blank in the muffle differs from one operation to another, the operation is oftentimes operated at reduced speed during practically the entire course of the pressing or, even, for the entirety of the course of the pressing.
The muffle with the blank already disposed therein is typically brought to a preheat temperature in a so-called preheat press oven. Following the placement of the muffle in the press oven, the relatively cold press stamp is traveled downwardly and the press stamp contacts, either via the ceramic sealing-off piston that, in reality, melts together with the ceramic blank or itself directly, the blank so that stresses occur at the same time, and as well, the heat expansion coefficient of the press stamp is regularly clearly greater than the heat expansion coefficient of ceramic.